A critical bug in Internet Explorer browser also affects Outlook 2003, it has been revealed.

The vulnerability can be triggered when Explorer or Outlook 2003 processes web-based graphics code written in the Vector Markup Language (VML). It was first reported Monday by researchers at Sunbelt Software.

Attackers have not yet begun exploiting the email attack, but a handful of websites now serve the code, and hackers have publicly posted software that exploits the vulnerability. Experts predict the code will be available on tens of thousands of websites within just a few days.

Initially, researchers thought that only Explorer was vulnerable to attacks, but Sunbelt has now concluded that Outlook 2003 users are also at risk. That's because researchers have discovered a way to execute malicious code without using scripting code, which would normally be blocked by Outlook. By embedding a machine-language "shellcode" program in the VML tags, researchers have been able run unauthorised software on systems running the latest version of Outlook 2003.

This has raised concerns because it means that some victims could have their PCs compromised with little or no user action. To attack Internet Explorer, hackers would first need to trick users into visiting a malicious website. But with an Outlook attack, it becomes much easier to target a victim. "All you have to do is send an HTML email and the user is hosed," said Eric Sites, Sunbelt's vice president of research and development.

Researchers at VeriSign's iDefense unit have also confirmed that some configurations of Outlook will launch the code with no user action, said Ken Dunham, the director of the iDefense Rapid Response Team. Users who have Outlook's Reading Pane enabled to read messages in HTML are particularly vulnerable to this attack, Dunham said. Outlook Express users do not appear to be at risk, said Sites.

Microsoft is advising users who want to protect themselves to set Outlook to read email messages in plain text format. Instructions describing how to do this can be found here. The software giant has put out an advisory on the issue and is rapidly working toward a fix for the problem.

Sunbelt has posted a workaround for the vulnerability.